XDSL Modems: Fundamentals and Flavors

2.1 The Simple DSL Transceiver

In analog data communication along the PSTN, a voice-band modem converts data from a piece of terminal equipment into electronic signals in the 200 Hz to 3.4 kHz frequency band. This allows the existing public network to transmit electronic data in the same way it traditionally would a human voice. In the early decades of data communication, this was not so much of a problem. However, as modems have evolved to transmit and receive data at ever-higher speeds, and as software has evolved to carry ever more complex forms of information, data communication presses up against the physical limitations of the copper medium. The bandwidth of 200-3400 Hz is simply too narrow to fit this data comfortably. The result is a communications bottleneck. Downloading a web page becomes an increasingly cumbersome process the more detailed its graphics are. Try to connect to a web page that features animation - or worse yet, video footage - and your computer will slow to a crawl.

DSL frees the end-user from the limitations of voice bandwidth, providing bandwidth measured in the hundreds of kilohertz and enabling communications at least 100 times faster than that available over pure POTS, while still allowing you to make phone calls while your PC or fax is transmitting or receiving. Let us examine a typical DSL modem to see how it accomplishes this.

Figure 2-1: A Typical DSL Transceiver Block Diagram

The DSL chip set includes both analog and digital components. Among the analog components are analog transmit and receiver filters, the Digital to Analog Converter (DAC), the Analog to Digital Converter (ADC), and the automatic gain device (to adjust the received signal level to that which is suitable to the input of the ADC).

The modulation/demodulation function of the DSL transceiver, the modem proper, is digital. Modulation defines the process of converting each successive data symbol vector - in this case, a DSL input bit - into a continuous time analog signal that represents the message corresponding to each successive group of bits. At the far end of the transmission, the receiving DSL unit converts these analog signals back into bit form, hence "demodulation." Subsumed within the modulation/demodulation function are such aspects of digital signal processing as echo cancellation, adaptive channel equalizing, symbol/bit conversion, timing recovery, constellation mapping. In the cases of Carrierless AM/PM (CAP) or Quadrature Amplitude Modulation (QAM) line codes, the modem also provides the digital shaping filter, while in the case of Discrete MultiTone (DMT) line code, the modem includes Fast Fourier Transform (FFT) and Inverse Fast Fourier Transform (IFFT).

This brings us to the other major digital function of the DSL chip set, coding/decoding - performed by a part of the transceiver prosaically-known as the encoder. The task of the encoder is to map data bits from a digital bit stream prior to modulation and transmission. The importance of coding varies depending on the flavor of xDSL in use. Earlier DSLs, such as IDSL and HDSL, require no coding at all. Later DSLs, ADSL for example, can use Reed-Solomon codes, trellis codes or both. In the most recent generation of DSL systems, HDSL2 being the prime example, coding forms a critical part of the DSL transceiver. The relationship of the encoder to the modulator in transmission appears below in Figure 2-2.

Figure 2-2: Transmitter of a digital transmission system

Besides the DSL chip set, but there are two other components our hypothetical DSL transceiver may contain. The first element is the hybrid circuit, an interface converter for conversion from four-wire, dual half-duplex to two-wire full-duplex. The second element is the POTS splitter, a low-pass filter that separates the voice channel out from the DSL communication spectrum. The POTS splitter thus allows you to use your phone line as a phone line while simultaneously using it for data communication via modem, fax machine, or other terminal equipment.

These, then, are the bare bones of an average DSL modem. The modem connects the customer premises to the local loop, the actual digital subscriber line. The digital signal may require regeneration while traveling along the local loop, a process carried out by repeaters. At the far end of the local loop lies the central office, the CO, where another DSL modem will pick up the digital transmission. A bird's eye view of this generic DSL architecture appears below in Figure 2-3.

Central Office - Building where local loops connect to transmission links.
LT: Line Termination - Building where local loops connect to transmission and switching equipment.
Local Loop - Telephone wire connecting the CO to the customer premises.
MDF: Main Distributing Frame - Wire cross-connection field used to any loop to any Central Office equipment.
NID: Network Interface Device - Point of demarcation between the customer installation and the telephone company outside wire.
NT: Network Termination - DSL Modem at the customer end of the local loop.
Repeater - Signal regeneration device located near the midpoint of a cable.
TE: Terminal Equipment - End user equipment such as a personal computer or a telephone.

Figure 2-3: Generic DSL Reference Model

As we will discover, however, terms like "average" and "generic" must be applied very carefully when referring to DSL technology. There exists a veritable alphabet soup of DSL modems, with more combinations of letters clumping together all the time. Let's grab our spoons and taste a few.

2.2 The Many Flavors of DSL

There are nearly a dozen different types or "flavors" of DSL modem currently in existence or in development. We will now examine each type with respect to its transmission capabilities and limitations.

2.2.1 IDSL

Between 1982 and 1988, the American National Standards Institute, better known as ANSI, developed the standards that defined IDSL (ISDN DSL) communications. IDSL functions on the Basic Rate Interface (BRI) model of ISDN - also known as 2B+D - which provides an overall data transmission rate of 144 KBPS. The two B (bearer) channels are circuit switched and can carry 64 KBPS of either voice or data in either direction. The D (data) channel carries control signals and customer call data in a packet-switched mode, operating at 16 KBPS. Remaining throughput is absorbed by Operational, Administrative, Maintenance and Provisioning (OAM&P) channels.

IDSL runs on a single pair of wires at a maximum distance of 18 kilofeet (kft) - roughly 3.4 miles/5.4 km. What differentiates IDSL from traditional ISDN is that ISDN requires connection through a CO voice switch. IDSL runs directly through xDSL equipment, rendering unnecessary the expensive upgrades to CO switches which ISDN otherwise demands. For this reason, IDSL is sometimes known as "BRI without the switch."

A variant on IDSL relies on the Primary Rate Interface (PRI) model, using a single D channel at 64 KBPS and some 23 to 30 B channels. In this variant, the channels are bonded to form higher bit rates, providing a theoretical capacity of up to 768 KBPS.

2.2.2 The HDSL Family: HDSL, SDSL, MSDSL and HDSL2

The search for a more cost-effective route to provide PRI bandwidths on local loops also led to the development of an entirely new flavor of DSL. AT&T Bell Laboratories and Bellcore developed the concept High-bit-rate DSL (HDSL) in late 1986. Prototype HDSL systems first appeared in 1989 and became commercially available three years later. In technical terms, HDSL is a dual-duplex repeaterless T1 technology. This means that HDSL transmits data symmetrically over two pairs of wire at the standard T1 data rate of 1.544 MBPS (each pair carrying 784 KBPS), an order of magnitude faster than IDSL. By itself, T1 transmission requires a repeater every 6000 feet to "clean up" and relay the digital signal to its destination, a requirement that often made such transmission too expensive for the average customer.

Figure 2-4: T1 Components

HDSL overcomes this limitation using a line code adapted from IDSL called 2B1Q (Two Binary, one Quaternary). The 2B1Q code compresses two binary bits of data into one time state as a four-level code. This doubles the effective range of T1 transmission from 6,000 to 12,000 feet without repetition, slicing the cost of T1 communication, with a side benefit of reducing crosstalk. In its original form, HDSL required addition of a third wire pair to bring the data rate up to E1 levels (2.048 MBPS).

The primary use of HDSL is to provide companies and individuals with Internet access to servers, but not just from clients. Additional applications include providing links on private campus networks with installed copper cable plant, video conferencing and distance learning applications, providing PRI for ISDN, extending central PBX to other office park locations, providing LAN extensions and connections to fiber rings and providing wireless system base station connections.

Figure 2-5: The HDSL Architecture

Multiple pairs of wires can prove troublesome when it comes to digitizing the analog local loop for residential service, as opposed to commercial premises. Using a single pair proves less troublesome. The result has been an offshoot or little brother of HDSL, the basic version of which runs at 784 KBPS, full-duplex on a single pair of wires. This flavor is known as SDSL, standing either for Symmetric DSL or Single-pair DSL, depending on the source. Since its introduction, SDSL has developed various incarnations, with the data rate varying inversely to the maximum distance. One proprietary form of SDSL is Multirate Symmetric DSL (MSDSL). To confuse matters further, MSDSL is also sometimes referred to simply as "MDSL," for Multirate DSL, an acronym shared by a form of Asymmetric DSL technology.

SDSL Data Rate
128 22,000 6.71
256 21,500 6.56
384 14,500 4.42
768 13,000 3.97
1,024 11,500 3.51

Table 2-1: SDSL Speeds and Distances

Another variation of HDSL, recently standardized, is HDSL2. Like SDSL, it functions on a single, full-duplex twisted pair. Unlike SDSL, it can transmit the full T1 (1.544 MBPS) or E1 (2.048 MBPS) at a distance of up to 12 kft without repeaters. The downside is that HDSL2 - designed for the T1/E1 leased line business market, rather than the residential market - does not include voice circuit support.

Subsequent to the printing of this book, a new flavor of HDSL, called SHDSL, has emerged. It is the first standardized multi-rate symmetric DSL and is designed to transport symmetrical data across a single copper pair at data rates from 192 KBPS to 2.3 MBPS or 384 KBPS to 4.6 MBPS over two pairs. Refer to the chart in Figure 2-9.

2.2.3 The ADSL Family: ADSL, MDSL, RADSL and Splitterless DSL

All the flavors of DSL we have examined thus far have one facet in common: they all have the same rate of data transmission downstream (from service provider to customer) as upstream (from customer to service provider). There are a number of applications for DSL, however, in which the data traffic downstream tends to be much heavier than requests for data sent back upstream. This is especially true for Video-on-Demand (VOD), but also holds true for Internet access (particularly on the World Wide Web) and LAN bridging. It follows that a DSL used for these applications could allocate bandwidth more efficiently were it able to transmit data asymmetrically, accelerating data transmission downstream at the cost of upstream transmission speed. This would have the additional benefit of reducing near-end crosstalk (NEXT).

This is the guiding concept behind Asymmetric DSL (ADSL). The early concept for ADSL originated in 1989, while HDSL was still in the prototype phase, under J.W. Lechleider and others at Bellcore. Stanford University and AT&T Bell Labs developed ADSL from concept to prototype between 1990 and 1992, with field technology trials beginning three years later. The International Telecommunications Union (ITU) gave determination to a set of ADSL recommendations in October 1998.

ADSL employs one of two modulation techniques, CAP and DMT. CAP is "Combined Amplitude Phase Modulation." DMT is "Discrete Multi-Tone Modulation." DMT has recently been adopted as the ADSL standard.

ADSL has a downstream transmission rate of between 1 and 9 MBPS, with an upstream transmission rate of between 64 KBPS and 1 MBPS and can operate at distances up to 18 kft. ADSL also allows the use of standard voice telephony in addition to data transmission, by the use of a POTS splitter. With the splitter, data transmission and POTS flow through the same line, with the digital transmission restricted to a frequency band above that of voice telephony. In physical terms, voice-band signals are attached to the red and green inside wires to the telephone, while wideband signals attach to the yellow and black inside wires to the customer's ADSL. A low-pass filter (LPF) for the voice wiring is placed at or near the customer premises' Network Interface Device (NID), while a high-pass filter is installed in the customer's ADSL modem-proper (ATU-R) for higher frequency data.

Subsequent to the printing of this book, new flavors of ADSL have emerged, including ADSL2+ and ADSL2++, capable of doubling the transmission speed of typical ADSL connections to 2.2 MHz and 4.5 MHz, respectively. Both are backwards compatible to ADSL and provide improved reach. Refer to the chart in Figure 2-9 for a comparison.

Figure 2-6: ADSL Reference Model

Figure 2-7: Conventional ADSL Configuration with Splitter

ADSL has already generated a number of offshoots, such as Medium-bit-rate DSL - MDSL, not to be confused with the symmetric DSL that sometimes goes by that acronym. MDSL evolved as a way to provide a less complex, less expensive ADSL modem. The tradeoff is speed. The downstream data transmission rate for MDSL is only 800 KBPS to 1 MBPS; its upstream rate, a mere 100 KBPS.

Another version of ADSL is Rate-Adaptive DSL (RADSL). In some situations, line conditions or sensitivity to environmental changes may interfere with operation at the assumed optimum speed. RADSL compensates for such hazards, adjusting the operating rate to the highest possible for the local loop. On average local loops, RADSL may have a downstream rate of 7 to 10 MBPS and an upstream rate of 512 to 900 KBPS. On long loops (18 kft or more), RADSL operates downstream at about 512 KBPS and 128 KBPS upstream. RADSL, like ADSL, makes use of a POTS splitter to separate ADSL from voice-band transmission.

For all the advantages the POTS splitter grants, however, it also carries the disadvantage of requiring the setup of extra premises wiring, as existing substandard wiring will degrade ADSL performance. Further, the shifting of ADSL transmission to higher frequency bands to accommodate POTS reduces ADSL data rates and loop reach. Splitterless DSL, recently standardized, solves this problem.

Splitterless DSL goes by a plethora of different trade names, including Commercial DSL (CDSL), Universal ADSL (UDSL or UADSL), DSL Lite and G.Lite (for ITU Recommendation G.992.2, which governs this flavor). To call this flavor "splitterless" is actually something of a misnomer. Rather than eliminating the need for a splitter altogether, it allows the line to be split at the CO end of the connection. This takes much of the burden off the customer, who can now have ADSL service merely by plugging an ADSL modem into a phone jack, without the need for extensive premises rewiring or splitter installation. This makes splitterless DSL both simpler and less expensive than earlier versions of ADSL. Now that it has become standardized, it is expected to become the dominant version. Splitterless DSL carries downstream data transmissions at 1 MBPS to 6 MBPS and upstream transmissions at 128 KBPS to 384 KBPS.


2.2.4 VDSL

Very-high-bit-rate DSL (VDSL) is the newest flavor of DSL technology and has been in development since late 1995. Unlike its various elder siblings, VDSL has the option of either symmetric or asymmetric transmission. The highest symmetric rate proposed would leave current HDSL modems in the dust, zipping data transmission along at 26 MBPS. The asymmetric rates currently under consideration vary from 13 MBPS downstream/1.6 MBPS upstream, to 26 MBPS downstream/3.2 MBPS upstream, to an incredible 52 MBPS downstream/6.4 MBPS upstream. The tradeoff for these fantastic transmission speeds is in distance. VDSL only has a service range of 1.5 to 4.5 kilofeet, restricting its usefulness. For this reason, VDSL technology is targeted for use as the last link in fiber in the loop (FITL), fiber to the curb (FTTC) and fiber to the neighborhood (FTTN) networks. Like ADSL, VDSL allows for the coexistence of digital and POTS transmission on the same twisted pair by use of a POTS splitter.

Figure 2-8: The VDSL Architecture

Figure 2-9: Sampling technologies and related bandwidths

This accounts for all the major, and quite a few of the minor, flavors of DSL modem currently on the market. However, there is one more device we require before our digital transmissions can make the leap from the CO to the end-user destination: the Digital Subscriber Line Access Multiplexer, or DSLAM.

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